This work involves the use recombinant DNA technology to construct bacterial and isolate phage containing actin sequences for use as a specific probe to measure actin messenger RNA in differentiating muscle cells. The (putative) genomic actin sequence containing fragments from a human phage bank were isolated and initial analysis of the specific sequence done by Southern gel analysis of restriction endonuclease fragments. The gene appears to be in at least two forms in the human genome both of which have an RI endonuclease site within coding fragments. Flanking the gene are middle-repetitive sequences. Concurrently, the actin protein organization and conformation in developing muscle cells is being investigated by electron microscopy and the isoforms of actin are being investigated by two-dimensional electrophoresis.